Dadun Community:http://hdl.handle.net/10171/231812015-08-02T20:25:14Z2015-08-02T20:25:14ZSteam oxidation of ferritic steels : kinetics and microstructurehttp://hdl.handle.net/10171/382312015-05-05T08:42:37Z2000-01-01T00:00:00ZTitle: Steam oxidation of ferritic steels : kinetics and microstructure
Abstract: The ferritic 2.25Cr–1Mo steel has been subjected to isothermal and non-isothermal oxidation treatments in water steam at
several temperatures ranging from 550 to 700 °C for up to 56 days. Under isothermal conditions this steel follows a parabolic
oxidation kinetics, with an activation energy of 324 kJ mol
–1
. This value corresponds to an apparent activation energy for the
global process, which includes both outward diffusion of Fe cations and inward diffusion of oxygen.
The oxidation products present in the oxide scales, which were characterised by X-ray diffraction and SEM, are in total agre
-
ement with the Fe-O phase diagram. Thus, magnetite is the most stable oxide at low temperatures and wustite starts to form
above 570 °C. Further studies of the effect of cooling rate have shown that wustite formed at 700 °C transforms into magne
-
tite during a slow cooling, whereas a rapid cooling inhibits this transformation to a certain extent.
For non-isothermal oxidation treatments consisting of a holding period at 550 °C followed by a single or double 4 hours
exposure at 700 °C, the oxidation process seems to occur in sequence, thus presenting an additive effect of the oxidation
treatments carried out at each temperature. This effect was observed both, for the type of oxide grown, and for the kinetics
of the process. Microscopic observations of the oxide scales formed after the various oxidation treatments revealed that the
oxide scales are constituted by sublayers of distinct microstructure and chemical composition changing from their surface to
the substrate interface.2000-01-01T00:00:00ZAnálisis termodinámico de la descarburación de aceroshttp://hdl.handle.net/10171/382302015-05-05T08:54:41Z2000-01-01T00:00:00ZTitle: Análisis termodinámico de la descarburación de aceros
Abstract: El control del contenido en carbono en aceros pulvimetalúrgicos durante el tratamiento de sinterización requiere plantear el
equilibrio y la cinética de la reacción del metal con la fase gaseosa. En este artículo se aborda el estudio termodinámico. En
la práctica industrial se suelen emplear diferentes atmósferas, que pueden contener CO, CO
2
, CH
4
ó H
2
. Con ayuda de pro
-
gramas de cálculos termodinámicos como Thermo-Calc, es posible realizar estimaciones complejas de equilibrio. Se ha com
-
probado que la representación del diagrama de fases de un acero en el espacio de coordenadas potenciales actividad de
carbono frente a 1/T facilita la selección de las atmósferas gaseosas de reacción. Se presenta también el modo de calcular
analítica y gráficamente la actividad de carbono en función de la relación de presiones parciales
p
(CO)/
p
(CO
2
) y
p
(CH
4
)/
p
(H
2
).
Como ejemplo, se ha estudiado el control de la descarburación en el hierro puro y en las aleaciones Fe-Cu y Fe-Cr.; It is necessary to study both the equilibrium and the kinetics of gas-solid reaction in order to control the carbon content of
powder metallurgy steels during sintering. In this paper a thermodynamic study is given. Several atmospheres are used in
industrial practice, that can contain CO, CO
2
, CH
4
or H
2
. With the aid of powerful software for thermodynamic calculations
as Thermo-Calc, it is possible to do estimations of complex equilibria. It has been proved that representations of the phase
diagram of a steel in the potentials space carbon activity vs. 1/T can help to select gas atmospheres. Here both a graphical
and an analytical method are presented for calculating carbon activity as a function of the partial pressures ratios
p
(CO)/
p
(CO
2
) and
p
(CH
4
)/
p
(H
2
).
As an example, the decarburation control in pure iron and in certain Fe-Cu and Fe-Cr alloys is studied.2000-01-01T00:00:00ZMeasurement of bone lengthening forres; an experimental model in the lambhttp://hdl.handle.net/10171/231782014-12-15T09:24:57Z1997-01-01T00:00:00ZTitle: Measurement of bone lengthening forres; an experimental model in the lamb
Abstract: To obtain the mechanical behaviour pattern of the lengthening process. DESIGN. IN VIVO: measurement of forces during bone engthening in lambs. BACKGROUND: A series of parameters of a mechanical and biological nature have a
bearing on all lengthening processes, and most of them are not fully understood.
METHODS: A strain-gauge-monitored unilateral fixator was designed and used to
obtain data about the changes which took place in the forces of elongation at a
rate of 1 mm/day in four lambs while a 3 cm progressive lengthening of the left
tibia was being performed, analysing how these forces behaved from day to day,
and how they changed in the course of a single day. RESULTS: The maximum forces
in all the animals each day occur after distraction, and the forces reach their
greatest magnitude between days 21 and 25 after surgery, attaining values of
slightly over 8 kg (40-50% of the animal's weight). The maximum daily force
starts to drop 1 h after distraction, and continues to decrease gradually
throughout the day until it reaches a value slightly greater than the initial
force on the previous day. CONCLUSION: This pattern is due to the distraction of
soft tissues which gradually adapt to their new situation, thereby reducing the
level of stress. RELEVANCE: In the daily bone lengthening procedure, the greatest
forces are produced in a short period of time immediately after lengthening. they
could be reduced to decrease pain in the patient and loads on the device by
performing lengthening over a greater number of steps or using dynamic equipment
able to absorb these forces.1997-01-01T00:00:00Z